1. Field of the Invention
The present invention relates to a multi-carrier transmission system using orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol, and a method for transmitting information, such as bit-loading parameters, in a multi-carrier transmission system.
2. Discussion of the Background
The demand for provision of multi-media and other bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL and VDSL. One of the more likely modulation systems for all these transmission schemes is a line code known as DMT (discrete multi-tone), which bears some resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4 kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, a number of transmission schemes have proposed the use of asymmetric schemes in which high data rates are transmitted in one direction only. Such schemes meet many of the demands for high bandwidth services, such as, video-on-demand but, in the long term, symmetric duplex systems will be required.
VDSL technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL:
CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passive NT configurations, CAP would use QPSK upstream and a type of TDMA for multiplexing (although CAP does not preclude an FDM approach to upstream multiplexing);
DMT; Discrete Multi-Tone, a multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA multiplexing strategy);
DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using Wavelet Transforms to create and demodulate individual carriers, DWMT also uses FDM for upstream multiplexing, but also allows TDMA; and
SLC; Simple Line Code, a version of four-level baseband signalling that filters the base band and restores it at the receiver, for passive NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
At the present time, bit-loading is used to vary the number of bits per sub-carrier, in so called discrete multi-tone, and OFDM-transmission systems. Information concerning channel properties, such as bit-loading and energy-loading information, is transmitted as a sequence of N numbers, where N is the number of the sub-carriers.
It is necessary, when using discrete multi-tone (DMT) systems, to transmit information concerning how the channel properties affect the transmission, for example, the number of bits that is to be transmitted per symbol. When using a large number of sub-carriers, a relatively large amount of information, relating to channel properties, has to be transmitted. It is, therefore, a requirement of such systems to provide an effective method for Transmitting such information.
It is an object of the present invention to provide, in a multi-carrier transmission system, a method for the effective transmission of information, such as, bit-loading information, i.e. a method in which the number of information bits required to be transmitted is reduced in comparison to known systems, thereby effecting a saving in transmission capacity.
It is another object of the present invention to provide a multi-carrier transmission system in which a information, such as, bit-loading information, can be effectively transmitted, i.e. by reducing the number of information bits required to be transmitted and thereby effecting a saving in transmission capacity.
According to a first aspect of the present invention, there is provided a multi-carrier transmission system in which channel information is transmitted between two transceivers using a plurality of sub-carriers, characterized in that each sub-carrier, or symbol, has a parameter associated therewith, and in that said transceivers are adapted to transmit said channel information as a sequence of a number of groups, in which each of said groups contains information concerning the number of adjacent sub-carriers which have the same value as said parameter, together with the actual value of the parameter.
Said parameter may have a plurality of discrete values and may be a bit-loading value, or a QAM constellation identifier.
In accordance with the invention, the number of information bits required to be transmitted is substantially reduced in comparison to known systems, thereby effecting a saving in transmission capacity.
In preferred arrangement for the multi-carrier transmission system said parameter may have a plurality of discrete values, and the number of information bits transmitted will be reduced if
P(m+n) less than Nxc3x97n
where P is the number of groups, the number of discrete values of said parameter is xe2x89xa62n, but  greater than 2nxe2x88x921, and the number of sub-carriers N is xe2x89xa62m, but  greater than 2mxe2x88x921.
In accordance with preferred system arrangement, set out in the preceding paragraph, the value of N may be equal to 1024, n may be equal to 4 and m may be equal to 10. Thus, if P equals 50 groups, the number of bits to be transmitted will be reduced by 83%. If, however, P is less that 200 groups, the number of bits to be transmitted will be reduced by at least 32%. The number of bits to be transmitted will be reduced by at least 66%, if P is less that 100 groups.
According to a second aspect of the present invention, there is provided a transmitter for a multi-carrier transmission system in which channel information is transmitted between two transceivers using a plurality of sub-carriers, characterised in that each sub-carrier, or symbol, has a parameter associated therewith, and in that said transmitter is adapted to transmit said channel information as a sequence of n groups, in which each of said n groups contains information concerning the number of adjacent sub-carriers which have the same value as said parameter, together with the actual value of the parameter.
In a preferred arrangement for the transmitter said parameter may have a plurality of discrete values and may be a bit-loading value, or a QAM constellation identifier.
According to a third aspect of the present invention, there is provided a transceiver for a multi-carrier transmission system, characterised in that said transceiver includes a receiver and a transmitter as outlined in preceding paragraphs.
According to a fourth aspect of the present invention, there is provided a multi-carrier transmission system including two transceivers as outlined in the preceding paragraph.
According to a fifth aspect of the present invention, there is provided, in an a multi-carrier transmission system in which channel information is transmitted between two transceivers using a plurality of sub-carriers, a method for transmitting channel information, characterised in that each sub-carrier, or symbol, has a parameter associated therewith, said method being characterised by the step of transmitting said channel information as a sequence of n groups, in which each of said n groups contains information concerning the number of adjacent sub-carriers which have the same value as said parameter, together with the actual value of the parameter.
In a preferred method of the present invention, said parameter may have a plurality of discrete values and may be a bit-loading value, or a QAM constellation identifier.
With the method of the present invention, the number of information bits required to be transmitted is substantially reduced in comparison to known systems, thereby effecting a saving in transmission capacity.
In accordance with the method of the present invention, said parameter may have a plurality of discrete values, and the number of information bits transmitted will be reduced if
P(m+n) less than Nxc3x97n
where P is the number of groups, the number of discrete values of said parameter is xe2x89xa62n, but  greater than 2nxe2x88x921, and the number of sub-carriers N is xe2x89xa62m, but  greater than 2mxe2x88x921.
In accordance with the method, set out in the preceding paragraph, the value of N may be equal to 1024, n may be equal to 4 and m may be equal to 10. Thus, if P equals 50 groups, the number of bits to be transmitted will be reduced by 83%. If, however, P is less that 200 groups, the number of bits to be transmitted will be reduced by at least 32%. The number of bits to be transmitted will be reduced by at least 66%, if P is less that 100 groups.
In accordance with the invention, a multi-carrier transmission system, transmitter, transceiver, or method, outlined in preceding paragraphs, may be characterised by the said transmission system being a DMT system.
Alternatively, in accordance with the invention, a multi-carrier transmission system, transmitter, transceiver, or method, outlined in preceding paragraphs, may be characterised by the said transmission system being a DMT-based VDSL system.